The beam is too deep Mr. Engineer 1

[ANE91]

12-foot span supporting two residential stories, (2)14” LVLs flush, joist bays are 12’ and 16’, contractor wants 9-1/4” instead. Deflection controlled. No chance of reconfiguring the floor plan to cut down the trib width or add interior supports. I can’t think of a way to make this work…5 or 6 LVLs feels like crazytown. Any creative ideas before I crush his dreams of a mildly taller ceiling?

I’m sure there’s plenty of other posts about this exact topic, but I wanted some contemporary feedback while I searched. Thanks y’all.

 

[SE2607]

Could the reason be related to providing rotational restraint at wood bearings? I know this can be done with a flange plate bolted to the wood, but that can be messy.

Rotational restraint? I'm assuming the original analysis considered the supports as pinned and free to rotate. I, too, like to have steel posts supporting steel beams, but, when I do, I weld the beam to the column (because, usually, there is no room for a cap plate) analyze the beam/columns as a frame. This not only reduces the beam size but also more correctly analyzes the column because, with rotation, there is moment.

 

[jerseyshore]

Rotational restraint? I'm assuming the original analysis considered the supports as pinned and free to rotate. I, too, like to have steel posts supporting steel beams, but, when I do, I weld the beam to the column (because, usually, there is no room for a cap plate) analyze the beam/columns as a frame. This not only reduces the beam size but also more correctly analyzes the column because, with rotation, there is moment.

9 out of 10 times I use a knife plate since there is no room for a cap plate. No welding required. Simple bolting.

 

[Ron247]

not good at shorter spans as the end reactions are typically very high.

XR250, Could you explain the correlation between short spans and very high end reactions. That has not been what I have seen over the years.

I am generally using flitch beams on remodel projects more than new-build. Contractors I work with prefer the flitchs over the steel beams.

 

[XR250]

What about detailing it so that it's shallower than the wood, and held 1/4" to 1/2" above the bottom of the wood plies? That's my go-to...curious if you've seen issues with that approach in the wild. I haven't gotten complaints, but then I wouldn't be surprised if they just spray painted a piece of plywood black to trick a city inspector...

These has been threads about this previously. I honestly have never seen an issue but doing the math on how the load gets moved around from steel to wood makes a brain hurt.

 

[XR250]

XR250, Could you explain the correlation between short spans and very high end reactions. That has not been what I have seen over the years.

I am generally using flitch beams on remodel projects more than new-build. Contractors I work with prefer the flitchs over the steel beams.

So in the OP's case, 2 floors at 14 ft. avg. trib, the end reaction would be about 7k. Moment would be about 20 ft-k so roughly a 3/4x9 flitch. Now use that flitch in a different case where it has a 20 ft. span (say supporting a roof) , it could only take about 350 PLF before it fails in deflection and the end reaction is 3.5k - much more palatable.

 

[SE2607]

9 out of 10 times I use a knife plate since there is no room for a cap plate. No welding required. Simple bolting.

Maybe I'm not visualizing it correctly, but wouldn't that connection induce moment into the column due to the eccentricity?

 

[Ron247]

Thanks XR, I see what you mean now.

"For a given beam section that is loaded to the max", the shorter span ones tend to support more total load because they are generally controlled by shear rather than moment/deflection. But that is true for even steel beams, not just flitch beams.

I was thinking why would 10' at 1,000 lbs per foot have higher reactions than 20' at 1,000 lbs per foot. You are saying the shorter beam could support a higher load magnitude.

 

[Respawn]

Not sure what your loads are. 7x wide might be the best option to avoid any steel. Anyway a PSL can work here? Also you stated "contractor wants".... What are his reasons? Is the Architect fine with using 14" leading to step in ceiling or it protruding down?

 

[jerseyshore]

Maybe I'm not visualizing it correctly, but wouldn't that connection induce moment into the column due to the eccentricity?

Very minor. I believe AISC lets you ignore any eccentricity as long as it's not an "extended" connection. A simple knife plate that's a few inches long with a single row of bolts isn't going to be an issue.

 

[XR250]

For a given beam section that is loaded to the max", the shorter span ones tend to support more total load because they are generally controlled by shear rather than moment/deflection. But that is true for even steel beams, not just flitch beams.

Form JS's post....
"We always show the flitch plates being shallower than the wood, but flush to the bottom. No way to transfer the shear at the end thru bolts into the columns with heavy flitches. Need the steel plates to bear directly on the PSL post and/or steel bearing plate."

 

[phamENG]

I guess I'm not usually loading these to the max - I use them for longer spans where deflection controls and the end reactions are manageable through bolts. Sometimes I'll split it multiple plates to get more shear planes.

 

[SE2607]

Very minor. I believe AISC lets you ignore any eccentricity as long as it's not an "extended" connection. A simple knife plate that's a few inches long with a single row of bolts isn't going to be an issue.

I understand AISC's position on shear tabs not required to be designed for the eccentricity, but what about the column? If the distance between the bolts and the end of the shear tab is 1-1/2", there is a 1/2" gap between the end of the shear tab and face of column and the column is 3" wide, then that's an eccentricity of 3-1/2". That doesn't seem insignificant to me.

 

[phamENG]

There have been many threads discussing the topic. The most recent was last June, I believe.